1. Field of the Invention
This invention relates to the manufacture of liquid transfer rolls such as ceramic coated printing rolls, and particularly to hard engraved or embossed surfaces on aluminum and other metal rolls used for transferring liquid such as ink in printing.
2. Description of the Related Art
It is a well established and accepted practice in the printing industry, and in other industries where process materials such as ink, varnish, and adhesives are transferred from one surface to another, to apply ceramic coatings to liquid carrier rolls fabricated from aluminum, steel and other suitable materials. Very hard and wear-resistant ceramic coatings such as refractory oxides or metallic carbides may be applied by thermal spray technologies. After application of the thermal sprayed coating, the roll surface is ground and/or polished to a very smooth finish and then engraved with a high power laser to create "inkwells" or cell patterns in the coating. These cells carry the printing ink or other liquid process materials. Such coatings have virtually revolutionized the printing industry over the past two decades. These ceramic coatings enable the transfer rolls to withstand the wear generated by the continuous scraping of a steel knife (doctor blade) utilized to maintain a uniform film of the liquid being transferred on the roll's surface. The ceramic coatings are equally advantageous in the transfer of adhesives, varnish and other liquids.
The thermal spray processes rely on the introduction of fine particles of the preferred ceramic into a high energy, high temperature gas stream. The powder particles are heated to plasticity and propelled onto the surface to be coated where they impact and form a mechanical bond with the substrate. Additional layers of the coating are applied until the desired coating film thickness has been achieved. Precision grinding and polishing techniques are then employed to create a smooth, dimensionally stable coating. Liquid transfer rolls are engraved at this point with laser equipment to form the ink-carrying (or other liquid-carrying) cells. To make a cell, the laser partially melts and vaporizes the ceramic coating it contacts at a discrete point or line to create an open channel, cell or hole, which becomes an inkwell (or container for other liquid). A by-product of the laser activity is a semi-molten slag, similar to volcanic lava, which forms around the hole and re-solidifies. It appears that this phenomenon occurs because the thermal spray coating consists of individual powder particles, some of which are not fully plasticized. This is the nature of the thermal spray process. The re-solidified ceramic is very hard and brittle. When the process roll is placed into service, pieces of the brittle ceramic can break off and become embedded in the roll surface or in the steel doctor blade. An embedded piece of hard ceramic can scratch or cut a groove in the working surface of the roll in a very short period of time, destroying the quality of the roll face and necessitating its replacement. The embedding problem has become more and more frequent as the cells have become smaller with the ever-increasing demand for higher print quality.
A second shortcoming of the thermal sprayed ceramic coating is that a roughened surface remains in the cell after the laser engraving operation. This rough surface makes cleaning of the roll (removal of ink or other material) extremely difficult and time consuming. Valuable time is lost and it is not uncommon to damage the cells during the cleaning process.
A desirable improved process would be one which has little or no ceramic re-melt or liquid flow on application of the laser to cut the ink-well patterns. Additionally, a desirable process would leave holes or cells having sharply defined patterns, having walls which are smooth and of the same texture as the surrounding area, to facilitate ink (and other material to be transferred) removal and cleanup as well as more precise patterns. Easier cleanup increases productivity and minimizes the chances of damage to the roll surface. And, a desirable process would be one which does not demonstrate a significant shortcoming of the thermal spray processes--the sometimes inadequate adherence of the coating to the roll substrate. This is an inherent difficulty with the thermal spray processes particularly for substrates, such as aluminum, having coefficients of thermal expansion considerably different from the ceramic coating.
Further, it would be desirable to have a process wherein the laser-produced cell patterns are imparted to the roll surface prior to application of the ceramic coating rather than after, since application of the laser after coating incorporates all the above possible defects and shortcomings, and results in vertical surfaces in the cells which are different in composition from the horizontal surfaces. Coating after engraving is not feasible with sprayed ceramics.
The reader will be interested in the disclosure of U.S. Pat. No. 5,616,229 to Samsonov and Hiterer, which proposes the formation of ceramic coatings of up to 300 microns within about 90 minutes through the use of an alternating current of at least 700 volts having a shaped wave (not the conventional sinusoidal form) which rises from zero to its maximum height and falls to below 40% of its maximum height within less than a quarter of its full alternating cycle, thereby causing dielectric breakdown, the alternating current being imposed on an electrolytic bath in which the metal subject is an electrode, the bath comprising initially an alkali metal hydroxide and in a later step including an oxyacid salt of an alkali metal, such as sodium tetrasilicate. While the '229 patent speaks of forming coatings on aluminum surfaces, the authors do not treat the possible use of such a coating process for the manufacture of aluminum process rolls, where the coating step is integrated with a laser system to engrave or emboss a three-dimensional pattern for holding liquid.
Laser engraving of hard-coated rolls is described in U.S. Pat. Nos. 4,794,680, 5,089,683, 5,093,180, and 5,143,578, which are incorporated herein, in their entirety, by reference.